This reverts commit aa731872.

As pointed out by Ben Hutchings, this change is not correct.

mdiobus_unregister() can't be called if the bus isn't registered yet,
however this change can result in situations which cause that to
happen.

Part of the confusion here revolves around the fact that the
callers of this module control registration/unregistration,
rather than the module itself.
Signed-off-by: David S. Miller <davem@davemloft.net>